In view of the adjustment of tool post angle during hobbing modified spur gear, the basic worm helicoids of hob is generated according to national standards. The model of the hobbing is established according to space engagement theory and actual processing conditions. The formulae for the transversal shape of workpiece gear and its tooth-profile and tooth-thickness deviations are derived. Several calculating examples are given. The results show that there are different zero (tooth-profile and tooth-thickness) deviation tool post angle for different modification coefficient. Finally, the approximate formulae for zero deviation tool post angles are given to be easy to use in engineering. The tool post angle adjusted according to the method can ensure precision of workpiece gear.